Not Your Parents' Carbon: A New Type of Crystalline Graphite

Pure carbon can take a great variety of forms. Diamond, carbon nano­tubes and graphene—the last the subject of the 2010 Nobel Prize in Physics—­all have unique physical and chemical qualities and applications to technology. Now evidence is mounting that there is yet another crystal structure to add to carbon’s catalogue of wonders: a material that could find applications in mechanical components whose hardness varies depending on the pressure to which they are exposed.

This new type of carbon was first observed in 2003, when researchers placed graphite, a stacking of chicken-wire-shaped networks of carbon atoms, under high pressure at room temperature. Under this “cold” compression, the graphite began to assume a hybrid form, between that of graphene and of diamond, but its exact nature was unknown.

Two computer simulation studies now suggest that cold-compressed graphite contains crystals of a structure called body-centered tetragonal, or bct, in addition to another type called M carbon. In bct, groups of four atoms are arranged in a square. The squares are stacked in an offset manner, and each square forms chemical bonds with four squares in the layers above and four below. A team led by Hui-Tian Wang of Nankai University in Tianjin, China, showed that during cold compression the transition to bct carbon results in a release of energy, which means it is likely to happen in the real world.

A Japanese and American team also conducted a simulation in which bct carbon produced x-ray patterns similar to those seen in the 2003 study. The match between the simulation and the experiment is quite good, says Wendy L. Mao of Stanford University, who was part of the 2003 discovery. Whether bct carbon exists or can be synthesized in its pure form “is still a task for experimentalists to test.”